This invention relates, in general, to vacuum deposition systems and, more particularly, to a novel means for securing a wafer or substrate to a rolling and orbiting planet in a vacuum deposition system.
The typical production deposition system used for depositing either a layer of a single element or a combination of elements on a substrate, usually consists of a vacuum chamber in which is contained a source of the material to be deposited and a substrate or target on which the source material is to be deposited. The system is usually under vacuum and, when the pressure within the vacuum chamber is reduced to the required level, the source material is then heated to the point of vaporization and deposition of the source material on the substrate or target occurs.
In an attempt to insure a uniform deposition of the source material where, for example, one desires to deposit a layer of aluminum on a semiconductor wafer, as many as seven or eight wafers may be mounted on a disc or planet. These wafers may range in diameter from about two inches to about four inches depending upon the requirements of the user. A plurality of planets are then mounted in rolling engagement with a pair of circular tracks in order to position the planets in a plane inclined toward the source. The source is usually located on the system axis and, traditionally, is positioned a few inches below the plane of the lower track on which the planets are rotating.
The planets are then caused to orbit about the system axis on the pair of circular tracks, while the source material is being vaporized and deposited on each of the semiconductor wafers. Thus, as each planet orbits the system axis, each wafer to be coated is caused to rotate about the axis of the planet to insure a uniform coating on each of the wafers. Systems of this sort are described in detail in U.S. patent application Ser. No. 89,858, filed Oct. 31, 1979, now U.S. Pat. No. 4,284,033, entitled "Means to Orbit and Rotate Target Wafers Supported on Planet Member" to E. H. del Rio and assigned to the same assignee as the subject application. Other systems for coating substrates are described in U.S. Pat. No. 3,643,625 to G. O. B. Mahl on Feb. 2, 1979, entitled "Thin-film Deposition Apparatus" and U.S. Pat. No. 4,010,710 to E. E. Williams on Mar. 8, 1977, entitled "Apparatus for Coating Substrates". These systems are cited as being typical of vacuum deposition devices.
Various planet configurations are utilized for holding the substrate during the deposition process and may have either a flat or dished configuration with apertures counter-bored therein to accept the wafers or substrates during the metallization process. In many instances, the wafers are retained by similarly shaped dished backing plates which contain springs or other holding devices in order to exert pressure on the individual wafers to hold them in place. Typically, the springs would be located at about the center of the wafer.
When rotating a planet in a vacuum system wheren the planet may contain as many as 20-24 wafers mounted thereon, it should now be obvious that the total force applied by the backing plate to secure each wafer can be considerable. Any misalignment of the backing plate, or any defective spring which protrudes further than the others represents a source of wafer breakage. In addition, some of the springs that remain properly aligned lose their tension after a certain period of use and become too weak to hold the wafer securely in place. These latter springs are difficult to detect due to the large total force required to secure the backing plate. It has been found that the generally accepted technique for determining if the assembled planet and associated backing plate has been properly loaded is to lift the assembly and shake it. This shaking operation will indicate which wafers are too loose and likely to fall out during the rotation and orbiting of the assembly about the system axis. Should there be any loose wafers that are likely to fall out and break, the assembly must be disassembled and the spring tension on the defective springs adjusted in order that all wafers be retained in its proper place. Despite these precautions, it has been found that springs lose their tension during a run, thereby allowing one or more wafers to fall out and break, thus enhancing the possibility that the system will jam. Should this occur, the deposition run will be useless and processing of the good wafers must be repeated. This results in unproductive down time for the machine and a lowered through-put.